<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="button" value="点我" onclick="on()"><br>
<input type="button" value="再点我" id="btn"><br>
<script>
    function on(){
        alert("我被点了")
    }

    document.getElementById("btn").onclick=function () {
        alert("我又背点了")
    }
</script>
</body>
</html>